Hvordan vi måler virkningerne af metan har betydning for klimapolitikken

Hvor effektiv er fremme af kødfattige diæter til at reducere drivhusgasemissioner sammenlignet med kulstofpriser, når effektiviteten af ​​afbødningspolitikker måles i forhold til metans langsigtede adfærd? Et internationalt hold af forskere undersøgte, hvordan fokus på enten kort- eller langsigtede opvarmningseffekter af metan kan påvirke klimabegrænsende politikker og kostomlægninger i landbruget.

I modsætning til de andre hoveddrivhusgasser (GHG) og især kuldioxid (CO₂ ), har metan (CH4) en kort atmosfærisk levetid (omkring 10 år). Dens opvarmende effekt er betydelig på kort sigt, men aftager på lang sigt. Afhængigt af den betragtede tidsskala kan metans bidrag til landbrugets emissioner og klimaændringer variere betydeligt. Dette har vigtige konsekvenser i udformningen af ​​globale klimaændringspolitikker for landbruget.

Baseret på fremskrivninger fra tre landbrugsøkonomiske modeller, er undersøgelsen netop offentliggjort i tidsskriftet Nature Food viser, hvordan forskellige værdiansættelser af metan, der afspejler enten et kort- eller langsigtet fokus, kan påvirke omkostningseffektiviteten af ​​afbødningspolitikker og fordelene ved kødfattige diæter.

Konventionelt evalueres klimapåvirkningen af ​​en bestemt sektor gennem dens årlige drivhusgasemissioner, typisk ved hjælp af det globale opvarmningspotentiale over en 100 års periode-metrisk ̶ GWP100 ̶, som estimerer ændringen i atmosfærisk energibalance som følge af en bestemt type drivhusgasemission. Men da drivhusgasemissioner rapporteres som CO₂ -ækvivalenter (som er et meget stabilt drivhusgas), kan GWP100 ikke fange, hvordan de relative påvirkninger af forskellige gasser ændrer sig over tid.

Den kortvarige karakter af metan-emissioner er uden tvivl blevet overset i de fleste vurderinger af emissionsreduktioner, der kræves fra landbrugssektoren for at nå klimamålene. Forfatterne undersøgte, hvordan forskellige værdiansættelser af metan påvirker rangeringen af ​​afbødningspolitikker i landbruget og dermed sektorens bidrag til global opvarmning.

"The study highlights the importance of methane abatement options to slow down the contribution of agriculture to global warming. Given the short atmospheric lifetime of methane, which not only delivers climate effects on a relatively short time horizon, but also contributes to climate change, this makes it an interesting and essential element in agricultural mitigation policy design," explains study coauthor Stefan Frank, a researcher in the IIASA Integrated Biosphere Futures Research Group.

The findings indicate that mitigation policies focused on methane's short-term impact lead to greater emission reductions and the authors highlight that focusing specifically on the short-term effects of methane will lead to larger reductions of emissions compared to policies that do not consider methane's short-liveness. Such stringent mitigation policies can even result in methane's contribution to climate change dropping below current levels (since the warming effect of methane disappears). In this respect, decreasing methane emissions have the same overall effect as CO₂ uptake or carbon capture and storage technologies.

The authors further highlight that the impact of low animal protein diets as a mitigation option strongly depends on the context in which it is occurring. If mitigation policies base themselves on metrics that reflect methane's long-term behavior (resulting in a lower relative valuation), methane emission intensity is not as greatly reduced by technical measures. Then, low meat diets appear as a more effective option to reduce emissions. If policies are less strict, reductions in meat consumption and consequently production in developed economies could therefore become an especially powerful mitigation mechanism.

"The methane specificities—by which we mean a significant warming effect and related mitigation potential in the short term and the "climate neutrality" of a stable level of emissions in the long term—deserve distinct treatment in climate mitigation policies. This has been also recognized in the Global Methane Pledge, announced by the US and the EU and supported by more than 100 countries, which represents a commitment to reduce particularly global methane emissions by at least 30 percent from 2020 levels by 2030," says coauthor Petr Havlik, who leads the Integrated Biosphere Futures Research Group at IIASA.

A combination of innovative production-side policy measures worldwide and dietary changes in countries with high calorie consumption per capita could achieve the most substantial emission reduction levels, helping to significantly reverse the contribution of agriculture to global warming.